Bad breath is caused by many factors. Normally, saliva helps wash away the natural buildup of bacteria in the mouth and on the tongue. Yet, when saliva sits on the back of the tongue and is digested by bacteria, it shows up as a white film on the tongue. This is a major source of bad breath. Bad breath is also caused by postnasal drip, a condition where the back of the tongue and throat accumulate high amounts of protein, which bacteria thrive on. Foods that are high in protein or acidity, such as fish, milk, cheese or coffee, also cause bad breath. Dieting or fasting, both of which involve a low intake of calories, causes bad breath by reducing the saliva in the mouth. Contrary to popular belief, stomach problems do not cause bad breath.